Mice vaccinaed with irradiated cercariae of S. mansoni develop resistance to challenge infection in the absence of overt pathology. In order to identify mechanisms of parasite rejection in this model, mice with genetic or induced immunodeficiencies have been analyzed and compared for their vaccine induced resistance and schistosome specific immune responses. The results of these studies suggest that in the mouse, vaccine induced immunity is dependent on both B and T lymphocytes and that the effector mechanism of parasite rejection in this model may involve both a humoral and cellular component. Neither complement nor mast cells appear to play a role in this effector mechanism. A strain survey has revealed a polymorphism in the ability of different inbred mice to respond to the vaccine and genetic experiments have indicated that the expression of high levels of immunity is inherited as a dominant trait. The genes involved in the control of resistance also effect the levels of antibody produced in response to the vaccine. One of these genes is linked to the mouse major histocompatibility.